1. Technical Field
The present disclosure relates to a communication system, a terminal, and a communication control method.
2. Description of the Related Art
In the 3rd Generation Partnership Project (3GPP), standardization of Mission Critical Push To Talk (MCPTT) is currently ongoing.
3GPP TS 22.179 v13.3.0, “Mission Critical Push to Talk (MCPTT) over LTE; Stage 1” describes prerequisites to formulating services and system details of the MCPTT. In 3GPP TS 22.179 v13.3.0, “Mission Critical Push to Talk (MCPTT) over LTE; Stage 1”, services using an on-network service, an off-network service, and both the on-network and off-network services are contemplated as MCPTT services.
The on-network service is a service using Group Communication System Enablers for LTE (GCSE_LTE; see, for example, 3GPP TS 22.468 v13.0.0, “Group Communication System Enablers for LTE (GCSE_LTE)” and 3GPP TS 23.468 v13.3.0, “Group Communication System Enablers for LTE (GCSE_LTE); Stage 2”). 3GPP TS 22.468 v13.0.0, “Group Communication System Enablers for LTE (GCSE_LTE)” and 3GPP TS 23.468 v13.3.0, “Group Communication System Enablers for LTE (GCSE_LTE); Stage 2” state that in communication using the GCSE_LTE, terminals (user equipment: UE) participating in a group service such as the MCPTT may perform unicast communication using an Evolved Packet System (EPS) bearer described in 3GPP TS 23.401 v13.5.0, “General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access” or 3GPP TS 36.300 v13.2.0, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2”, or may perform multimedia broadcast and multicast service (MBMS) communication using an MBMS bearer described in 3GPP TS 23.246 v13.3.0, “Multimedia Broadcast/Multicast Service (MBMS); Architecture and functional description”.
The off-network service is a service using proximity services (ProSe: see, for example, 3GPP TS 22.278 v13.2.0, “Service requirements for the Evolved Packet System (EPS)”, 3GPP TR 23.713 v13.0.0, “Study on extended architecture support for proximity services”, 3GPP TS 23.303 v13.2.0, “Proximity-based Services (ProSe); Stage 2”, and 3GPP TS 36.321 v12.6.0, “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification”), which is device-to-device direct communication. The ProSe communication is sometimes called sidelink communication.
Examples of the service using both the on-network and off-network services include a service using a UE-to-network relay described, for example, in 3GPP TS 22.179 v13.3.0, “Mission Critical Push to Talk (MCPTT) over LTE; Stage 1”, 3GPP TS 23.303 v13.2.0, “Proximity-based services (ProSe); Stage 2”, and 3GPP TS 23.179 v13.0.0, “Functional architecture and information flows to support mission critical communication services; Stage 2”. The service using a UE-to-network relay refers to a service in which a UE located in a coverage of a base station (eNB: eNode B) functions as a relay UE relaying communication with a UE (remote UE) located outside the eNB coverage and the EPS. In this service, the off-network service using the ProSe communication is provided between the relay UE and the remote UE, while the on-network service using the GCSE_LTE is provided between the relay UE and an Evolved Packet Core (EPC; hereinafter, also described as Packet Core). This relay UE functions as a Layer 3 relay, namely, a Layer 3 router for the remote UE.
UEs of users provided with the MCPTT service belong to a group of the MCPTT service, as described in 3GPP TS 22.179 v13.3.0, “Mission Critical Push to Talk (MCPTT) over LTE; Stage 1”. The number of groups to which each UE belongs may be more than one.
In the MCPTT service, floor control is required to be supported, which permits only a user with a floor granted in the group to talk. The time to talk with the floor granted may be restricted. In an emergency and other similar situation, this service is required to permit pre-emption. In case of the pre-emption, an ongoing MCPTT service is suspended and the floor is assigned to the pre-emption. 3GPP TR 23.779 v13.0.0, “Study on application architecture to support Mission Critical Push To Talk over LTE (MCPTT) services” and 3GPP TS 23.179 v13.0.0, “Functional architecture and information flows to support mission critical communication services; Stage 2” disclose examples of registration with MCPTT service groups, the floor control, architecture of the MCPTT service including resource allocation to a network, and signaling.
As stated in 3GPP TS 23.179 v13.0.0, “Functional architecture and information flows to support mission critical communication services; Stage 2”, the above-mentioned unicast communication is used for uplink media data (such as audio and video; referred to as “data” hereunder) sent from the UE to an MCPTT server (to be described later), while for downlink data sent from the MCPTT server to the UE, the unicast communication, the MBMS communication, or both communications are available for use.